The present invention relates to a novel process for the preparation of 3,3'- or 3,4'-diaminobenzophenone.
3,3'-Diaminobenzophenone and 3,4'-diaminobenzophenone are useful as monomers for the production of heat-resistant high-molecular compounds, intermediates for the production of agricultural chemicals, pharmacological compounds and dyes, particularly as starting materials for the production of polyamides and polyimides.
Heretofore, these diaminobenzophenones have been prepared by reducing the corresponding dinitrobenzophenones. For example, 3,3'-diaminobenzophenone has been prepared by reducing 3,3'-dinitrobenzophenone in the presence of a tin compound in a large amount of concentrated hydrochloric acid [L.H. Kloron et al., J. Org. Chem., 23, 351(1958)]. 3,4'-Diaminobenzophenone has been prepared by reducing 3,4'-dinitrobenzophenone in the presence of a tin compound [I. Moyer. Hunsberger et al., J. Am. Chem. Soc., 71, 2637(1949)].
However, it is very difficult to industrialize these processes, because there has been no method of advantageously preparing 3,3'- or 3,4'-dinitrobenzophenone as the starting material. For example, 3,3'-dinitrobenzophenone may be prepared by nitrating benzophenone, but the resulting product is composed of a mixture containing various isomers or the like. In order to isolate the desired 3,3'-dinitro compound, large amounts of solvents must be used and purification by recrystallization must be repeated [E. Barmatt et al., J. Chem. Soc., 125, 767(1924)]. For this reason, the yield of 3,3'-dinitrobenzophenone is greatly lowered, and complicated stages are required for the recovery of solvents used for the purification and for the treatment of residue.
Recently, it has been proposed a process in which the nitration of benzophenone is conducted by using a large amount of oleum to selectively nitrate the meta-position. However, this process has problems in the disposal of a large amount of waste acid and materials for the apparatus [A. Onopchenko et al., J. Org. Chem., 46, 5014(1981)].
3,4'-Dinitrobenzophenone may be prepared, for example, by a process in which 4-nitrobenzyl alcohol is reacted with nitrobenzene to obtain 3,4'-dinitrodiphenylmethane which is then oxidized with chromic acid [P.J. Montagne, Ber., 49, 2293-2294 (1916)]; a process in which diphenylacetic acid is nitrated with fuming nitric acid to obtain 3,4'-dinitrodiphenylacetic acid which is then oxidized with chromic acid [I. Moyer Hunsberger et al., J. Am. Chem. Soc., 71, 2635-2639(1949)]; and a process in which 4-nitrobenzophenone is nitrated [Vernon, Li Bell et al., J. Org. Polymer, Chem., 14, 2277(1976)].
However, these processes have disadvantages in that very complicated reactions must be conducted, large amounts of by-products such as isomers must be removed, purification by recrystallization must be repeated and the disposal of waste acid and waste metals requires much cost.
Further, the separation of the tin compound used in the reduction of the dinitrobenzophenones is troublesome. Care must be taken lest trace amounts of the metal should remain. Much cost and effort are required for the disposal of waste metal and waste acid to prevent them from causing environmental pollution.
Therefore, the conventional processes of preparing dinitrobenzophenones and reducing them to diaminobenzophenones are industrially very unfavourable from the viewpoints of economy and environmental protection. Thus an improved process for the preparation of 3,4'-diaminobenzophenone, from which disadvantages associated with the prior art are eliminated has been eagerly seeked.